Panic with TOC extension

[MrZoidberg]

I'm using goldmark-toc extension to add a table of contents for my simple document, but the goldmark-markdown renderer panics.

Here is my markdown:

# Data API - DataAPI - Software Detailed Design

## Introduction

### Purpose


### External content


### External image

Here is my code:

import (
    markdownRndr "github.com/teekennedy/goldmark-markdown"
    "github.com/yuin/goldmark"
    "github.com/yuin/goldmark/extension"
    "github.com/yuin/goldmark/parser"
    "go.abhg.dev/goldmark/toc"
)

renderer := markdownRndr.NewRenderer()
markdown := goldmark.New(
    goldmark.WithExtensions(&toc.Extender{
        Compact:  true,
        MinDepth: 2,
        MaxDepth: 3,
    }),
    goldmark.WithParserOptions(parser.WithAutoHeadingID()),
    goldmark.WithRenderer(renderer),
)

var buf bytes.Buffer
if err := markdown.Convert([]byte(input), &buf); err != nil {
    return "", fmt.Errorf("Error rendering markdown: %v", err)
}

Error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1029f7270]

goroutine 1 [running]:
github.com/teekennedy/goldmark-markdown.(*Renderer).Render.func2({0x10315dfa8, 0x140004a0a20}, 0x1)
	/Users/mmerk/go/pkg/mod/github.com/teekennedy/goldmark-markdown@v0.4.1/renderer.go:106 +0x70
github.com/yuin/goldmark/ast.walkHelper({0x10315dfa8, 0x140004a0a20}, 0x1400069ef30)
	/Users/mmerk/go/pkg/mod/github.com/yuin/goldmark@v1.7.8/ast/ast.go:505 +0x4c
github.com/yuin/goldmark/ast.walkHelper({0x10315dd68, 0x140004a0990}, 0x1400069ef30)
	/Users/mmerk/go/pkg/mod/github.com/yuin/goldmark@v1.7.8/ast/ast.go:511 +0x1f8
github.com/yuin/goldmark/ast.walkHelper({0x10315eae8, 0x140004a0000}, 0x1400069ef30)
	/Users/mmerk/go/pkg/mod/github.com/yuin/goldmark@v1.7.8/ast/ast.go:511 +0x1f8
github.com/yuin/goldmark/ast.Walk({0x10315eae8, 0x140004a0000}, 0x1400069ef30)
	/Users/mmerk/go/pkg/mod/github.com/yuin/goldmark@v1.7.8/ast/ast.go:500 +0x3c
github.com/teekennedy/goldmark-markdown.(*Renderer).Render(0x140004a0ab0, {0x103155bc8, 0x140000ae510}, {0x140004fc1b0, 0x81, 0x90}, {0x10315eae8, 0x140004a0000})
	/Users/mmerk/go/pkg/mod/github.com/teekennedy/goldmark-markdown@v0.4.1/renderer.go:105 +0x1e0
github.com/yuin/goldmark.(*markdown).Convert(0x14000395d80, {0x140004fc1b0, 0x81, 0x90}, {0x103155bc8, 0x140000ae510}, {0x0, 0x0, 0x0})
	/Users/mmerk/go/pkg/mod/github.com/yuin/goldmark@v1.7.8/markdown.go:118 +0x100
main.(*MarkdownEngine).Render(0x1400069f960, {0x140004fc120, 0x81})
	/Users/mmerk/Code/redacted/design-doc-tool/main.go:316 +0x1d4
main.genFinalDocument({0x16da674cb, 0x30}, {0x102a95d9e, 0x18}, {0x102a9031b, 0xf}, {0x102a8f10b, 0xd}, 0x140003ad2c0, {{0x10315a160, ...}})
	/Users/mmerk/Code/redacted/design-doc-tool/main.go:177 +0xe64
main.main()
	/Users/mmerk/Code/redacted/design-doc-tool/main.go:74 +0xb24